The invention relates to actuating mechanisms and in particular to such mechanisms for use with parabolic reflector dishes.
Parabolic reflector dishes are used to collect and focus energy carried by waves. Examples of the waves include radio waves for communications and solar radiation which is focused to produce heat for power generation or to drive a chemical reaction.
Dishes are usually mounted on a structure with horizontal and vertical rotation axes providing altitude and azimuth tracking. It is known that placing the horizontal axis away from the centre of the dish places the dish closer to the ground when the dish is horizontal. With this configuration the vertical rotation axis is usually a kind of relatively simple turntable. The horizontal axis typically is more complicated. The actuating mechanism for rotating the dish about the horizontal axis is usually a hydraulic ram or a screw jack similar to that disclosed in U.S. Pat. No. 2,096,050. Both these mechanisms tend to be expensive and so are relatively small to keep their size and cost down. The small size means the mechanism must be placed close to the horizontal axis, which cause small movements of the mechanism to produce large movements of the dish which makes accurate tracking difficult. Placing the mechanism close to the axis also produces large forces on the horizontal axis itself, the structure which supports it and the mechanism, requiring them to be of high strength and therefore cost.
One method used to overcome the small sizes of current actuating mechanisms is to provide a xe2x80x98walking ramxe2x80x99 system which provides both altitude and azimuth tracking. This is achieved with a series of alternate extensions and contractions of appropriately mounted hydraulic rams. This system is however complicated and therefore expensive to manufacture and difficult to control.
According to the present invention, an actuator comprises:
a first generally cylindrical member;
a second generally cylindrical member received at least partially within, and co-axial with, the first generally cylindrical member;
a generally helical guideway provided on one of the first and second generally cylindrical members; and,
a plurality of guide means provided in a generally corresponding helical arrangement on the other of the first and second generally cylindrical members and being arranged to run along the helical guideway,
in which relative rotation of the first and second generally cylindrical members about their common axis causes the guide means to run along the helical guideway to cause relative axial movement between the first and second generally cylindrical members.
With the actuator of the present invention, the elongation or contraction of the actuator is achieved by the relative rotation between the two parts. Accordingly, this allows very accurate control of the change in length of the actuator, since a large rotational movement results in a relatively small change in the length of the actuator. Furthermore, the actuator gives a high mechanical advantage, and therefore a relatively low force is able to overcome a much higher force resisting the change in length of the actuator. This allows the actuator to move heavy objects. Therefore, the present invention gives an actuator which is reliable, accurate and efficient. The actuator according to the present invention is also less expensive to manufacture than other known actuators.
The actuator of the present invention can be made more easily and more cheaply than prior art actuators. In particular, it is preferred that the guideway is formed as a separate member to the cylinder. The formation of a separate guideway which is attached to the cylindrical member, for example by welding, is much easier and cheaper than providing a cylinder having a thick initial wall, and forming a guideway into the wall.
In one example of the present invention, the helical guideway is provided on the inner surface of the first generally cylindrical member, and the plurality of guide means are provided on the outer surface of the second generally cylindrical member. In an alternative arrangement, the helical guideway is provided on the outer surface of the second generally cylindrical member and the plurality of guide means are provided on the inner surface of the first cylindrical member.
Preferably, the helical guideway is in the form of a channel. In this case, the guide means are arranged to run inside the channel.
Advantageously, the guide means include a first set of guide means which contact the base of the channel. This ensures the two generally cylindrical members remain generally co-axial. Advantageously, the guide means include a second set of guide means which engage the side walls of the channel. This prevents undesired axial movement between the two generally cylindrical members.
Preferably, the guide means are rotatable guide means such as wheels or rollers. These guide means are rotated as they run along the helical guideway. This results in low friction between the members acting against rotation, whilst ensuring close contact between the members to reduce any play, and thereby prevent jamming.
Where the helical guideway is a channel, it is preferred that a first set of rotatable guide means are arranged with an axis of rotation generally parallel to the base of the channel, and that the rotatable guide means have a width smaller than the width of the channel. The axis of rotation may also be generally perpendicular to the flanges or side walls of the channel. The first set of rotatable guide members are arranged to contact and run along the base of the helical guideway channel. Additionally, it is preferred that a second set of the rotatable guide means are arranged with an axis of rotation generally parallel to the flanges. This axis of rotation may also be generally perpendicular to the base of the channel. Such rotatable guide means are arranged to contact and run along the side walls of the channel. The second set of rotatable guide means have a diameter less than the spacing between the opposed side walls of the channel.
It is beneficial that the second set of guide means are slightly offset from the helical path of the helical guideway such that some of the second set of guide means contact and run along one side wall of the channel, and the others of the second set of guide means contact and run along the opposite side wall of the channel. With this arrangement, when there is a change between a compressive and a tension force on the actuator, some of the second set of rotatable guide means are in contact with the side wall of the channel in the direction of the force and thereby prevent undesired longitudinal movement between the two generally cylindrical parts of the actuator.
In an alternative arrangement of the present invention, the generally helical guideway is formed as a projection from the side wall of the respective generally cylindrical member. In this case, it is preferred that one set of guide means are rotatable about an axis generally parallel to the side wall of the generally cylindrical member, and in use runs along the side wall of the generally cylindrical member. In this case, a strip, typically of metal, may be provided on the side wall of the cylinder along which the guide means run. This protects the cylindrical member from wear. A second set of rotatable guide means may be provided which are rotatable about an axis generally parallel to the side faces of the projection, and which means are arranged to run along the side faces of the projection.
In this case, it is advantageous that some of the second set of guide means contact and run along one side face of the projection and others of the second set of guide means contact and run along the other side face of the projection. In this way, relative axial movement between the two generally cylindrical parts of the actuator is prevented when the force applied to the actuator changes from a compressive to a tension force and vice versa.
In all cases, some, or preferably all, of the components including the guide means and helical guideway, are preferably formed of durable and advantageously corrosion resistant material. This helps prevent deterioration of the components which is especially important as these cannot be protected efficiently in other ways due to their relative movement.
It is preferred that the radial position of the guide means are adjustable. This ensures centring of the first and second generally cylindrical members with respect to each other, and ensures that the guide means are in the optimum position to contact the helical guideway. This in turn ensures smooth running of the guide means on the guideway and thereby prevents jamming. The radial position of the guide means may be achieved by providing the guide means on a threaded shaft, in which case the distance between the side of the generally cylindrical member on which the guide means are mounted and the guide means themselves can be adjusted by rotation. In this case, the threaded shaft may be threaded directly into the side wall of the generally cylindrical member, and therefore the spacing of the guide means can be adjusted by rotation of the shaft. Alternatively, nuts may be provided on the threaded shaft, the nuts being provided on either side of the side wall of the generally cylindrical member to sandwich the side wall, and in this case adjustment of the nuts on the threaded shaft will adjust the effective length of the shaft and hence the spacing of the rotation guide member from the side wall of the generally cylindrical member.
In a preferred aspect of the present invention, further guide means may be provided on the generally cylindrical member on which the helical guideway is provided, these guide means being arranged to contact the side wall of the other generally cylindrical member. This further assists in maintaining the coaxial arrangement between the generally cylindrical members and thereby prevents jamming and ensures smooth operation. These guide means are also preferably rotatable.
Advantageously, further generally cylindrical members may be provided which are connected to the first or second cylindrical member by a similar arrangement of generally helical guide means and a plurality of guide members which are arranged in a generally corresponding helical fashion. With this arrangement, any desired number of coaxial components can be arranged in telescopic fashion to achieve any desired overall length of actuator.
The actuator according to the present invention may advantageously be used for movement of a dish, such as a parabolic reflector dish for the collection and/or focusing of energy. Due to the accurate control of the actuator, and the good mechanical advantage, the actuator of the present invention may be provided close to the axis about which the dish is required to turn. However, the reduced cost of the actuator of the present invention compared to prior actuator means that larger actuators can be formed for the same cost as smaller prior actuators, and therefore the actuator can be provided further from the axis about which the dish is to rotate. This enables a reduction in the force required to turn the dish, and as the actuation is further from the axis, allowing greater accuracy and control and reducing the strength and therefore cost of axis and the structure supporting it.